Techniques for protecting passive optical networks

1. A method for performing a protection in a passive optical network (PON), comprising: forming a protection maintenance link between an active optical line terminal (OLT) and a standby OLT; forming a synchronization link between the active OLT and the standby OLT; computing a base differential distance value; continuously measuring round trip time (RTT) values by the active OLT using the protection maintenance link; periodically sending at least RTT values calculated by the active OLT to the standby OLT over the synchronization link; and computing, by the standby OLT, a new RTT value based on at least an RTT value measured by the active OLT and a standby differential distance value, when a switch-over action is triggered, thereby allowing the standby OLT to serve optical network units (ONUs) in the PON without performing a ranging process.

2. The method of claim 1 , wherein the base differential distance value is a differential distance of each of the active OLT and the standby OLT from a designated ONU in the PON.

3. The method of claim 2 , wherein the designated ONU is a collocated ONU coupled to each of the active OLT and the standby OLT.

4. The method of claim 2 , wherein computing the base differential distance value comprises: activating the designated ONU; measuring a base active RTT value between the active OLT and the designated ONU; measuring a base standby RTT value between the standby OLT and the designated ONU; exchanging the base standby RTT value and the base active RTT value between the standby OLT and the active OLT; and determining the base differential distance value to be a difference between the base active RTT value and the base standby RTT value.

5. The method of claim 4 , wherein computing the base differential distance value comprises computing the base differential distance value during installation of the PON or when a new OLT is added to the PON.

6. The method of claim 4 , further comprising: measuring, by the standby OLT, the standby differential distance value, the standby differential distance value being a differential distance between the standby OLT and the designated ONU measured at a specific time.

8. The method of claim 7 , wherein the new RTT value is a sum of the standby differential distance value and the RTT value measured by the active OLT.

9. The method of claim 1 , wherein upon performing a switch-over, the standby OLT becomes the active OLT and the active OLT becomes the standby OLT.

10. The method of claim 3 , further comprising utilizing an indication signal transmitted by the active OLT and received by the collocated ONU to indicate a status of at least one of a standby link and an active link.

11. The method of claim 1 , wherein computing the new RTT value comprises computing the new RTT value upon receiving the RTT value measured by the active OLT.

12. The method of claim 1 , further comprising transmitting switch-over trigger signals over the synchronization link.

13. The method of claim 1 , further comprising establishing the synchronization link over the protection maintenance link.

14. A duplex passive optical network (PON), comprising: a first optical line terminal (OLT) serving as an active OLT; a first optical network unit (ONU) collocated to the first OLT; a second OLT serving as a standby OLT; a second ONU collocated to the second OLT, the first our and the second OLT being configured to exchange at least round trip time (RTT) measures and PON related information over a synchronization link; and an optical splitter configured to connect the first OLT and the second OLT and their collocated first ONU and second ONU to a plurality of ONUs.

15. The duplex PON of claim 14 , wherein the first OLT is configured to control the second ONU and the second OLT is configured to control the first ONU.

16. The duplex PON of claim 14 , wherein each of the first OLT and the second OLT is configured to include a database for maintaining the RTT measures and PON related information.

17. The duplex PON of claim 14 , further comprising a protection maintenance link established from the first OTL and the first ONU to the second OLT and the second ONU, the protection maintenance link being configured to be used for continuous measurement of differential distance values.

18. The duplex PON of claim 17 , further comprising a synchronization link established over the protection maintenance link.

19. The duplex PON of claim 17 , wherein a switch-over from the active OLT to the standby OLT occurs without performing a ranging process.

20. The duplex PON of claim 19 , wherein when the switch-over occurs, the standby OLT computes a respective new RTT value from the standby OLT to each of the plurality of ONUs, the respective new RTT value being a sum of an RTT value periodically measured by the active OLT and a standby differential distance value, the standby differential distance value being a differential distance between the standby OLT and the first ONU.

21. The duplex PON of claim 19 , wherein prior to performing the switch-over, the standby OLT computes a respective new RTT value from the standby OLT to each of the plurality of ONUs, the respective new RTT value being a sum of an RTT value periodically measured by the active OLT and a standby differential distance value, the standby differential distance value being a differential distance between the standby OLT and the first ONU.

22. A duplex passive optical network (PON) having a network side and a subscriber side connected by at least one fiber-optic cable, the duplex PON comprising: a first optical line terminal (OLT) and an associated first subscriber optical network unit (ONU) disposed on the network side of the duplex PON; a second OLT and an associated second subscriber ONU disposed on the network side of the duplex PON, the first OLT and the second OLT being configured to exchange round trip time (RTT) measures over a link; and a plurality of subscriber ONUs disposed on the subscriber side of the duplex PON, wherein the second OLT is configured to compute an RTT value based on the RTT measures and a standby differential distance value related to a differential distance associated with the first OLT and the second OLT when a switch-over action is triggered, thereby allowing the second OLT to serve the plurality of ONUs without performing a ranging process.